1. Field of the Invention
This invention relates to a method of and an apparatus for making a stencil by thermally perforating a thermoplastic resin film of heat-sensitive stencil material by a thermal head or the like, and to a heat-sensitive stencil material. More particularly, this invention relates to improvement in shape of perforations, printing quality and stencil making speed.
2. Description of the Related Art
Methods of making a heat-sensitive stencil are broadly divided into a method in which the resin film side of the heat-sensitive stencil material is brought into close contact with an original bearing thereon an image formed of a carbon-containing material and the resin film is perforated by heat generated by the image upon exposure to infra-red rays and a method in which the resin film of the heat-sensitive stencil material is imagewise perforated by two-dimensionally scanning the resin film side of the heat-sensitive stencil material with a device such as a thermal head having an array of micro heater elements. The former method will be referred to as xe2x80x9can analog stencil making methodxe2x80x9d and the latter method will be referred to as xe2x80x9ca digital stencil making methodxe2x80x9d, in this specification. At the present, the digital stencil making method is prevailing over the analog stencil making method since the former does not require carbon in the original and permits easy image processing.
When the stencil is made by the digital stencil making method, it is preferred that the perforations be discrete by pixel, and be uniform in shape and degree of penetration so that the thin lines and/or edges of the printings show rims faithful to the original, the solid portions of the printings has a sufficient density and the amount of ink to be transferred to each printing sheet can be well controlled not to cause offset (the phenomenon the ink on the surface of a first printed sheet stains the back side of a second printed sheet superposed on the surface of the first printed sheet).
On the other hand, in order to meet the recent demand for higher image quality, highly fine or high resolution thermal heads such as of 400 dpi or 600 dpi have been in wide use as the thermal device for thermally perforating the stencil material. Such high resolution thermal devices are generally lower than low resolution thermal devices in the maximum temperature they can provide. Accordingly, in order to perforate the stencil material in a given size with the high resolution thermal device, the stencil material should be more sensitive to perforation than when it is perforated by the low resolution thermal device. Further, since the number of perforations (pixels) increases as the resolution increases, it is preferred that the time required to form each perforation be shortened, that is, each perforation be formed at a higher speed. Thus, physical properties of the resin film, the structure of the thermal head, and the method of controlling the thermal head for meeting these demands have been searched for.
The thermoplastic resin film for the heat-sensitive stencil material produces shrinkage stress when heated by a heat source such as a thermal head and is perforated by shrinkage. In order to improve sensitivity to perforation of the heat-sensitive stencil material, there has been proposed thermoplastic resin film having a specified heat shrinkage factor as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-125190 or thermoplastic resin film having a specified heat shrinkage factor and a specified heat shrinkage stress as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 7(1995)-52573 and 7(1995)-68964. However, in these patent publications, the heat shrinkage factor or the heat shrinkage stress is specified on the basis of measurement of the heat shrinkage factor or the heat shrinkage stress when the film is heated several to several tens of minutes, which is very long as compared with the time for which the film is heated in the actual perforation. Further, the measurement is static and does not reflect the actual perforation. Further, though the heat shrinkage factor or the heat shrinkage stress measured by, for instance, TMA (thermo-mechanical analysis) under a macroscopic and quasi-static condition where the area to be heated is not smaller than several millimeters (mm) and the temperature change is 10xc2x0 C./min or so has been reported, the behavior of the perforations under a microscopic and dynamic condition in the actual stencil making process where the area to be heated by the thermal head or the like is several tens of micrometers (xcexcm) and the temperature change is 1xc2x0 C./xcexcs or so has not been reported. Thus the reported heat shrinkage factor or heat shrinkage stress does not conform to the actual perforation.
Further, conventionally, discussion on the perforation in the stencil making process has been made not on the basis of behavior of perforations in course of perforation but on the final state of perforations. In such discussion, physical properties of the resin film and the structure of the thermal head, and the method of controlling the thermal head are generally discussed in order to control the final size and shape of the perforations and the TMA data on the film is employed only to indicate the sensitivity to perforation. Accordingly, the properties of the film concerning to the degree to which the perforations are discrete by pixel and the shape of the perforations is stabilized are generally incompatible with the sensitivity to perforation of film and the speed at which the film is perforated. That is, when a film can be perforated so that the perforations are well discrete and uniform in shape, the film is less sensitive to the perforation and takes a long time to perforate. Naturally the opposition is also true. Accordingly, in the actual design of a stencil making system, a plurality of kinds of thermoplastic resin film are prepared, the sensitivity to perforation of each kind of film is determined by repeating experiments or TMA measurements, and one of the kinds of film which is most close to a target sensitivity is selected.
The general data on the heat shrinkage factor and heat shrinkage stress do not always conform to the evaluation of film obtained in the actual design of a stencil making system with respect to, for instance, discreteness and uniformity of shape of the perforations, the sensitivity to perforation and the perforating speed. As described above, this is because the TMA data and the like are obtained under a macroscopic and quasi-static condition whereas the actual perforation in the actual stencil making process is effected under a microscopic and dynamic condition. Further, it is difficult to read from the TMA data the performance of the film representing the perforating speed, the stability of the shape of perforations and the like except the sensitivity to perforation. Even about the sensitivity to perforation, it is difficult to estimate the difference in the sensitivity to perforation between film samples which are slightly different from each other, for instance, in TMA curve since it is actually impossible to prepare a variety of film samples which are different from each other in one or more particular factor such as the TMA curve with the other factors held to be the same. Accordingly, when a suitable kind of resin film is to be selected, stencils must be actually made using a variety of resin film samples, which adds to the development cost.
As described above, information obtained as a characteristic value in the stencil making experiments is only on the size and shape of the perforations at the time the perforations are completed. Accordingly, it has been very difficult to know, without experience and sense, how the physical properties of the resin film should be changed on the basis of the result of experiment in order to obtain a desirable form of perforation, which has been made difficult development of new products and improvement of the performance of the products. Unsatisfactory design of the performance of the resin film can result in the case where the sensitivity to perforation and perforating speed are too poor to obtain a high-resolution stencil under a practical condition though the perforations are discrete and substantially uniform in shape or in the case where the perforations are not discrete and not uniform in shape though the sensitivity to perforation and perforating speed are satisfactory.
Thus, it has been impossible to develop, on the basis of conventional data experimentally obtained, a method of and an apparatus for making a stencil by thermally perforating a thermoplastic resin film of heat-sensitive stencil material, and a thermoplastic resin film for heat-sensitive stencil material in which demands for uniformity in shape of perforations, sensitivity to perforation and perforating speed are all satisfied.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and an apparatus for making a stencil by thermally perforating a thermoplastic resin film of heat-sensitive stencil material, and a thermoplastic resin film for heat-sensitive stencil material in which perforations can be discrete and uniform in shape, and sensitivity to perforation and perforating speed are high.
In accordance with a first aspect of the present invention, there is provided a method of making a stencil by thermally forming perforations arranged in both a main scanning direction and a sub-scanning direction in a thermoplastic resin film of heat-sensitive stencil material by the use of a heat source which is heated through supply of energy, wherein the improvement comprises that supply of energy to the heat source is cut when a time interval not shorter than 50% and not longer than 100% of an estimated perforating time lapses from the time at which supply of energy to the heat source is started, the estimated perforating time being a time interval expected to be necessary for a perforation to be produced by the heat of the heat source and to be enlarged to a desired size as a final size as measured from the time at which supply of energy to the heat source is started.
The desired size as a final size is a size in which the perforation is to be formed when enlargement of the perforation is ended, and will be sometimes referred to as xe2x80x9ca target sizexe2x80x9d, xe2x80x9ca target diameterxe2x80x9d, or xe2x80x9ca target areaxe2x80x9d, hereinbelow.
It is preferred that the estimated perforating time be a time t2 represented by formula       B          A      ⁢              xe2x80x83            ⁢              exp        ⁡                  (                      Ct            2                    )                      =      4    100  
when a graph of the diameter of the perforation against the time from the time at which supply of energy to the heat source is started is regressed on an exponential function   A  -      B          exp      ⁡              (        Ct        )            
wherein A, B and C are positive constants.
It is preferred that the target diameters of the perforations in the main scanning direction and the sub-scanning direction be set not smaller than 45% and not larger than 80% of the scanning pitches in the respective directions.
Further it is preferred that the target area of the perforations be set not smaller than 20% and not larger than 50% of the product of the scanning pitches in the main scanning direction and in the sub-scanning direction.
In accordance with a second aspect of the present invention, there is provided an apparatus for making a stencil comprising a heat source which is heated through supply of energy, a heat source control means which supplies energy to the heat source and a scanning means which scans a thermoplastic resin film of heat-sensitive stencil material with the heat source to thermally form perforations arranged in both a main scanning direction and a sub-scanning direction in the thermoplastic resin film, wherein the improvement comprises that the heat source control means cuts supply of energy to the heat source when a time interval not shorter than 50% and not longer than 100% of an estimated perforating time lapses from the time at which supply of energy to the heat source is started, the estimated perforating time being a time interval expected to be necessary for a perforation to be produced by the heat of the heat source and to be enlarged to a desired size as a final size as measured from the time at which supply of energy to the heat source is started.
It is preferred that the estimated perforating time be a time t2 represented by formula       B          A      ⁢              xe2x80x83            ⁢              exp        ⁡                  (                      Ct            2                    )                      =      4    100  
when a graph of the diameter of the perforation against the time from the time at which supply of energy to the heat source is started is regressed on an exponential function   A  -      B          exp      ⁡              (        Ct        )            
wherein A, B and C are positive constants.
It is preferred that the heat source control means sets the target diameters of the perforations in the main scanning direction and the sub-scanning direction to be not smaller than 45% and not larger than 80% of the scanning pitches in the respective directions.
It is preferred that the heat source control means sets the target area of the perforations to be not smaller than 20% and not larger than 50% of the product of the scanning pitches in the main scanning and sub-scanning directions.
In accordance with a third aspect of the present invention, there is provided a thermoplastic resin film for stencil material which is scanned by a heat source, which is heated through supply of energy, in both a main scanning direction and a sub-scanning direction and is thermally formed with perforations arranged in the main scanning and sub-scanning directions in the thermoplastic resin film, wherein the improvement comprises that
the heat shrinkable properties of the thermoplastic resin film are such that the time interval from the time at which supply of energy to the heat source is cut to the time at which enlargement of the perforation is stopped is not shorter than 0% and not longer than 100% of the time interval from the time at which supply of energy to the heat source is started to the time at which supply of energy to the heat source is cut.
It is preferred that the time at which enlargement of the perforation (will be referred to as xe2x80x9cthe enlargement stopping timexe2x80x9d, hereinbelow) is stopped be set to be a time t2 represented by formula       B          A      ⁢              xe2x80x83            ⁢              exp        ⁡                  (                      Ct            2                    )                      =      4    100  
when a graph of the diameter of the perforation against the time t from the time at which supply of energy to the heat source is started is regressed on an exponential function   A  -      B          exp      ⁡              (        Ct        )            
wherein A, B and C are positive constants.
Though the time at which enlargement of the perforation is stopped is strictly the time at which enlargement of the perforation in all the directions is stopped, the time may be taken for the purpose of simplicity as the time at which enlargement of the perforation in both the main scanning direction and the sub-scanning direction is stopped.
With reference to FIG. 5, xe2x80x9cthe diameter of the perforationsxe2x80x9d is defined as follows. That is, in a perforation 21, the diameter of the perforation 21 in a given direction is a length 25 of an orthographic projection of the inner periphery (a boundary defined by a dark region of the inner slope of the rim to be described later in a bright-field image obtained through an optical microscope) of the rim 23 (an annular thickened part generated by thermal perforation) of the perforation 21 onto a straight line 24 parallel to the given direction.
The xe2x80x9carea of the perforationxe2x80x9d is the area of the part 22 (FIG. 5) circumscribed by the inner periphery of the rim.
These inventors have found a method of evaluating perforation from a novel point of view. That is, we observed the phenomenon that a small perforation was formed and enlarged with time when the thermoplastic resin film of the stencil material was brought into contact with the heat source such as a thermal head by the use of a system which could take an image in a microscopic field of view of the order of xcexcm at a high speed of xcexcs. The result is shown in FIG. 6. In FIG. 6, the ordinate represents the diameter of the perforation and the abscissa represents the time from the time supply of energy to the heat source is initiated. From FIG. 6, we have found that perforation occurs in the following four stages.
In the first stage, the thermoplastic resin film is heated by a heater element (heat source) of a thermal head the temperature of which is the highest at the center thereof and is lowered toward its periphery. The temperature of the film is the highest at a part in contact with the center of the heater element and as the distance from the part in contact with the center of the heater element increases, the temperature of the film lowers. When the temperature of the film exceeds a shrinkage initiation temperature at which the film starts to shrink, shrinkage stress, which tends to reduce the distance between any two points on the film, is generated and accordingly, tension is produced in any point of areas which are not lower than the shrinkage initiation temperature. The direction of the tension is substantially perpendicular to (just perpendicular to if thermal shrinkage is isotropic) isothermal lines on the film. On the other hand, where the temperature of the film is sufficiently low, no shrinkage stress is generated. Accordingly, resin of the film is moved away from the highest temperature point of the film as it slides down the temperature gradient.
In the second stage, an initial small perforation is generated near the highest temperature point of the film.
In the third stage, the outer periphery of the initial small perforation is pulled outward by tension from outside, whereby the perforation is enlarged (growth of the perforation by shrinkage stress). The outer periphery of the perforation is pulled outward and increases its volume taking in resin on its path, whereby the rim is formed.
In the fourth stage, the heater element is de-energized and its temperature lowers. As the temperature of the heater element lowers, the temperature of the film in contact with the heater element lowers, and when the temperature of the film becomes lower than the shrinkage initiation temperature, no tension acts on the rim and the shape of the perforation is fixed (end of the perforation). The diameter or the area of the perforation as measured in this stage will be referred to as the diameter or the area of the perforation xe2x80x9cin the final statexe2x80x9d, hereinbelow.
Thus we have found that the aforesaid incompatible requirements, that is, discreteness of the perforations, stability in shape of the perforations, sensitivity to perforation of the stencil material and high speed perforation, can be balanced at a high level by setting in a certain range the ratio of the time interval from the time at which supply of energy to the heat source is started to the time at which it is cut to the estimated perforating time out of the various parameters obtained from the perforation size versus energizing time curve.
That is, the aforesaid incompatible requirements can be balanced at a high level by cutting supply of energy to the heat source when a time interval not shorter than 50% and not longer than 100% of the estimated perforating time lapses from the time at which supply of energy to the heat source is started. When supply of energy to the heat source is cut before 50% of the estimated perforating time lapses from the time at which supply of energy to the heat source is started, sensitivity to perforation deteriorates and the perforations cannot be formed at a satisfactory speed. Whereas, when supply of energy to the heat source is cut after 100% of the estimated perforating time lapses from the time at which supply of energy to the heat source is started, the perforations cannot be discrete and at the same time the shape of the perforations becomes unstable.
Further, when the estimated perforating time is set to be a time t2 represented by formula       B          A      ⁢              xe2x80x83            ⁢              exp        ⁡                  (                      Ct            2                    )                      =      4    100  
when a graph of the diameter of the perforation against the time from the time at which supply of energy to the heat source is started is regressed on an exponential function   A  -      B          exp      ⁡              (        Ct        )            
(A, B and C are positive constants), the estimated perforating time can be determined without affected by accuracy in measuring the diameter of the perforation.
Further, when the target diameters of the perforation in the main scanning direction and the sub-scanning direction, that is, the diameters in the main scanning direction and the sub-scanning direction to which the perforation is expected to be enlarged after cut of the energy supply, are set not smaller than 45% and not larger than 80% of the scanning pitches in the respective directions, the amount of ink transferred through the stencil obtained can be such that offset can be avoided in solid parts while necessary density is ensured, and thin character parts can be sufficient in width and density.
Further, when the target area of the perforation, that is, the area to which the perforation is expected to be enlarged after cut of the energy supply, is set to be not smaller than 20% and not larger than 50% of the product of the scanning pitches in the main scanning and sub-scanning directions, the amount of ink transferred through the stencil obtained can be such that offset can be avoided in solid parts while necessary density is ensured, and thin character parts can be sufficient in width and density.
When the heat shrinkable properties of the thermoplastic resin film for the stencil material are such that the time interval from the time at which supply of energy to the heat source is cut to the time at which enlargement of the perforation is stopped is not shorter than 0% and not longer than 100% of the time interval from the time at which supply of energy to the heat source is started to the time at which supply of energy to the heat source is cut, the perforations can be discrete, the shape of the perforations can be stabilized, an excellent sensitivity to perforation can be ensured and a high perforating speed can be ensured. That the time interval from the time at which supply of energy to the heat source is cut to the time at which enlargement of the perforation is stopped is shorter than 0% of the time interval from the time at which supply of energy to the heat source is started to the time at which supply of energy to the heat source is cut is that enlargement of the perforation is stopped before supply of energy to the heat source is still continued. Such a phenomenon occurs when energy is kept to be supplied to the heat source longer as compared with the time required to perforate the thermoplastic resin film or when the shrinkage initiation temperature is low and the heat shrinkage stress is large to such an extent that enlargement of the perforation is stopped by the time temperature increase in the heat source becomes slow while the heat source is being energized. When the time interval from the time at which supply of energy to the heat source is cut to the time at which enlargement of the perforation is stopped is shorter than 0% of the time interval from the time at which supply of energy to the heat source is started to the time at which supply of energy to the heat source is cut, the shape of the perforations is apt to fluctuate though sensitivity to perforation and perforating speed can be high. Whereas when the former time is longer than the latter time, sensitivity to perforation and perforating speed cannot be high.
When the enlargement stopping time is set to be a time t2 represented by formula       B          A      ⁢              xe2x80x83            ⁢              exp        ⁡                  (                      Ct            2                    )                      =      4    100  
when a graph of the diameter of the perforation against the time t from the time at which supply of energy to the heat source is started is regressed on an exponential function   A  -      B          exp      ⁡              (        Ct        )            
(A, B and C are positive constants) the enlargement stopping time can be determined without affected by accuracy in measuring the diameter of the perforation.
The values of the diameter and the area of the perforations are not as measured in the thermoplastic film laminated on the porous support sheet (to form a heat-sensitive stencil) but as measured in the thermoplastic film by itself. This is because it is very difficult to observe the state of perforation and to measure the diameter and/or the area of the perforation in a state where the thermoplastic film is laminated on the porous support sheet. However, the state of perforation (the diameter and/or the area of the perforation) as measured in the thermoplastic film by itself has a high correlation with that as measured in the thermoplastic film laminated on the porous support sheet. FIGS. 7 and 8 show the correlation. In FIG. 7, the ordinate represents the diameters of the perforations in the final state when a heat-sensitive stencil material (a thermoplastic film and a porous support sheet laminated together) is perforated under various conditions and the abscissa represents the diameters of the perforations in the final state when the same thermoplastic film as that employed in the heat-sensitive stencil material is perforated by itself under the same conditions. The correlation coefficient of the graph shown in FIG. 7 is 0.913. In FIG. 8, the ordinate represents the areas of the perforations in the final state when a heat-sensitive stencil material (a thermoplastic film and a porous support sheet laminated together) is perforated under various conditions and the abscissa represents the areas of the perforations in the final state when the same thermoplastic film as that employed in the heat-sensitive stencil material is perforated by itself under the same conditions. The correlation coefficient of the graph shown in FIG. 8 is 0.9319. Thus it will be understood that the state of perforation in the thermoplastic film by itself can represent the state of perforation in the heat-sensitive stencil material comprising the thermoplastic film laminated with a porous support sheet.